1. Field of the Invention
The present invention relates generally to semiconductor devices and manufacturing methods and, more particularly, the present invention relates to a semiconductor device having both an npn transistor and one or more other devices which employ a plurality of polycrystalline silicon resistors on a common semiconductor substrate.
2. Description of the Related Art
In order to improve characteristics of bipolar transistors, a decrease in base resistance is important from the viewpoint of reduction of noise, improvement of the maximum frequency f.sub.max, low power consumption, and others. In most bipolar transistors, the base outlet electrode is made of a polycrystalline silicon (Si) film. To reduce the base resistance of such a bipolar transistor using a base electrode in the form of a polycrystalline Si film, it is effective to decrease the sheet resistance of the polycrystalline Si film forming the base outlet electrode.
An effective method for decreasing the sheet resistance of a polycrystalline Si film is to increase the crystalline grain size and to improve the crystallographic properties. An effective method therefor is to break the crystal of the polycrystalline Si film made on a semiconductor substrate to change it into an amorphous state by ion implantation and to thereafter re-crystallize it by annealing for a long time at a low temperature.
It may be possible to use the above-mentioned method to decrease the sheet resistance of a polycrystalline Si film forming the base outlet electrode of a bipolar transistor. In most cases, however, the polycrystalline Si film forming the base outlet electrode is made simultaneously with other outlet electrodes of other devices. Therefore, the above method may degrade the reliability and the yields of semiconductor devices, depending on elements and electrodes formed therein, due to damage by ion implantation for changing polycrystalline Si films into an amorphous state and centralized stress caused by large-sized crystalline grains.